Haemostasis is referred to as the natural balance of maintaining the fluidity of the blood in the vascular system and preventing excessive blood loss subsequent to blood vessel injury by rapid formation of a solid blood clot. After vascular damage, contraction of the vessels and platelet adhesion occur immediately followed by aggregation of the platelets, activation of the coagulation cascade and finally also of the fibrinolytic system. Haemostatic abnormalities can lead to excessive bleeding or thrombosis, both life-threatening situations.
A series of antiplatelet agents have been developed over the past several years based on different mechanisms of action. The most widely used agent in antiplatelet therapy is aspirin, which irreversibly inhibits cyclooxygenase-1 and thereby affecting the thromboxane pathway. Although not optimally efficacious, treatment with aspirin remains the standard therapy against which new therapeutics are compared and judged.
Other drugs like the phosphodiesterase inhibitors dipyridamole and cilostazol, as well as the vitamin K antagonists (warfarin), are marketed but do not show all desirable features for such drugs. Three intravenously applicable, potent GPIIb/IIIa receptor antagonists (abciximab, eptifibatide, and tirofiban) blocking platelet aggregation are available on the market. Besides, some orally active GPIIb/IIIa antagonists (e.g. sibrafiban, xemilofiban or orbofiban) have not been successful in clinical development so far.
Adenosine 5′-diphosphate (ADP) is a key mediator in platelet activation and aggregation interfering with two platelet ADP receptors P2Y1 and P2Y12.
Antagonists of the platelet ADP receptor have been identified and display inhibition of platelet aggregation and antithrombotic activity. The most effective antagonists known so far are the thienopyridines ticlopidine, clopidogrel and CS-747, which have been used clinically as antithrombotic agents. It could be shown that these drugs, via their reactive metabolites, irreversibly block the ADP receptor subtype P2Y12.
Some P2Y12 antagonists like AR-C69931MX (Cangrelor) or AZD6140 have reached phase II clinical studies. These inhibitors are selective platelet ADP receptor antagonists, which inhibit ADP-dependent platelet aggregation, and are effective in vivo.
Piperazino-carbonylmethylaminocarbonyl-naphtyl or -quinolyl derivatives have been described as ADP receptor antagonists in WO 02/098856 and WO 2004/052366.